With the development of the display technology, thin film transistor liquid crystal display (TFT-LCD) devices have become the most common display devices. As one of main types of flat display devices at present, the TFT-LCD devices have become important display devices of modern IT and video products.
Thin film transistors play a very important role in the active matrix display technology. Specifically, in an active matrix liquid crystal display device, each sub-pixel is provided with a thin film transistor, so that each sub-pixel may operate independently and be less susceptible by other sub-pixels. As shown in FIGS. 1 and 2, most existing thin film transistors have a bottom-gate structure, which comprises a gate electrode 11, a gate insulation layer 12, a semiconductor layer 13, and a source electrode 14 and a drain electrode 15 arranged sequentially from bottom to top, and the source electrode 14 and the drain electrode 15 are arranged in a same layer. The gate electrode 11 is connected to a scanning line 110, and the source electrode 14 is connected to a data line 140. In the thin film transistor having the above structure, a second metal layer (i.e., a layer where the source electrode 14 and the drain electrode 15 are arranged) covers the semiconductor layer.
In actual design, the source electrode 14 and the drain electrode 15 are relatively narrow in width. In addition, in order to reduce a risk of disconnection in an etching process, a line-width of a climbing portion of the drain electrode 15 (i.e., a portion which covers an edge of the semiconductor 13) is generally increased locally. In this way, an overlapping area of the second metal layer and a first metal layer (i.e., a layer where the gate electrode 11 is provided) is increased, and therefore parasitic capacitance between the first metal layer and the second metal layer is increased. Increased parasitic capacitance results in the greater capacitance coupling effect and the signal delay effect, which affects the display effect of the liquid crystal display device and reduces the display quality thereof.